1. Field of the Invention
Process for producing thin layers of large-surface, arched elements, especially cathode-ray-tube displays, by centrifugal coating.
2. Background Art
Usually, the outside surface of the display of a picture tube is highly polished and has very high electrical resistance. The bright, smooth surface causes often disturbing reflections, and the high electrical resistance results in an electrostatic charging of the display surface during the operation of the tube. To avoid these drawbacks, it is known to equip the surface of the display antistatically or antireflectively by applying one or more thin layers, for which numerous methods are known. It is common to apply, e.g., on the display surface, an alcoholic solution of silicon-organic compounds, especially silicon alcoholates, optionally together with titanium-organic compounds to match the index of refraction, in a thin layer which, after drying and baking, form an SiO.sub.2 film, which exhibits antistatic and antireflective properties. Once in a while, several layers, optionally with different indicates of refraction, are also applied after respective intermediate drying and then baked on together. Quite excellent antireflective effects can be produced especially with several layers.
In the design of the layers, it is especially important that the latter exhibit a uniform thickness over the entire surface of the object to be coated. As a coating process for producing thin layers of large-surface, arched elements, especially the centrifugal coating is used. In this process, the coating solution is fed to the object to be coated, distributed by rotation of the object to be coated and any possible excess is centrifuged off at the borders. The centrifuging process is a simple process and also works very quickly, but different layer thicknesses can occur especially in the outside and corner areas of large-surface displays, which can cause undesirable effects, e.g., interference.
To produce a uniform layer thickness, it is known, e.g., from EP 286 129 A, to place the display surface to be coated pointing downward in an obliquely arranged drum, which has an upward opening. The display surface is then sprayed while rotating with the coating solution. During the rotation, the display surface is direct with a hot-air blower with a part of the formed liquid film in and around the center of the display by targeted blowing with hot air. Such a process is comparatively expensive. From JP 2-12736 A, a process is known in which the display to be coated is used with the surface to be coated pointing upward into a trough whose holding plates for the display are configured so that they form practically a continuation of the display surface. Further, it is described in this patent specification to provide two diagonally placed, very high bars running up to approximately the center, which rotate synchronously with the picture tube during the coating. By these arrangements, a uniform layer thickness of the applied layer is to be achieved. But these processes do not always work satisfactorily, especially in the case of large display surfaces.